This invention generally relates to radiation imaging devices and apparatus therefor and is particularly directed to the provision of a beam collimator which, while exhibiting utility in various radiological environments, is particularly useful in association with computer-assisted tomographic imaging equipment.
Radiographic imaging techniques and apparatus have undergone evolutionary advancement in recent years. By most accounts, the most promising development in the radiographic imaging field can be found in the provision and refinement of computer-assisted tomographic equipment and techniques which now enables the practitioner to non-invasively obtain detailed information relative to the location and condition of organs and other tissues within a patient's body, such information taking the form of a radiographic image of a thin cross-section or "slice" of the area of the body under consideration.
Previously, medical X-ray images have followed the basic radiographic process utilizing a stationary flat plate of film or a fluoroscopic screen fixed in place behind the patient. With this basic process, an X-ray source impinges upon the patient and the variable transmission of the X-ray beam through the patient is registered by radiation-sensitive material such as the film. This basic radiographic process has been and is now subject to certain disadvantages. Specifically, subtle differences occurring naturally in tissue radiation transmission and/or absorption cannot be detected due to the low sensitivity of the radiation-responsive imaging material, and due to scattered radiation of the X-ray beam while passing through the patient. Furthermore, and as can be appreciated, spatial or tomographic information cannot be obtained from such a basic radiographic process in that images of objects disposed in superposition to one another will confusingly overlap on the radiation-responsive material or film and, in many instances, be undetectable due to the inability of the material to distinguish between minor differences in radiological subject contrast.
A more advanced approach specifically calculated to provide spatial information involved efforts which were directed toward producing an in-focus image of a single preselected plane through an object by blurring out shadow images produced by structure on all planes except the preselected plane. This was typically accomplished by a combined motion of either the source and the recording medium or the object and the recording medium which rendered the shadow image from one plane only as a stationary image on the recording medium. Other efforts were directed toward the production of stereoscopic radiographs, again in an attempt to provide spatial information, and all as is well-documented in the literature. These efforts and processes have met with limited clinical success, and generally have not provided images of acceptable quality.
The development of computer-assisted tomography has eliminated most of the major problems and disadvantages associated with the prior art radiographic processes, so as to clearly reveal the internal organs and tissues of the body in cross-section instead of superimposed on one another, this tomographic technique utilizing a series of X-ray exposures made from different angles and taken axially through a "slice" or thin cross-sectional area of a patient. Specifically, in the computer-assisted tomographic process, a narrow X-ray or Gamma ray beam is transmitted transversely through a section of the patient anatomy and is detected by a high-efficiency radiation detector. A series of individual measurements of transmitted radiation is made about the subject so as to obtain multiple views of the "slice" in section. By virtue of these views, a large amount of information and data is acquired as to the differences in radiological subject contrast of the cross-section in question. This information or data constituting individual measurements at many angles about the subject then provides the input by which X-ray or Gamma ray attenuation coefficients can be calculated in a computer and the image of the cross-section of the patient anatomy actually reconstructed. Thus, a computer-assisted tomographic scanner obtains, by mathematical reconstruction, a transverse sectional image from transmitted radiation projection data, all as is well known.
From an apparatus standpoint, and as shown in FIG. 1 of the appended drawings, a computer-assisted tomographic system generally consists of the following basic components. A collimated X-ray source 10 is provided designed to ideally deliver a very narrow beam or beams of photons 12 through a "slice" 14 in a patient 16. Scintillation or gas detectors 18 are disposed opposite the collimated X-ray source 10. Relative motion between the collimated X-ray source 10, the scintillation or gas detectors 18, and/or the patient 16 is typically achieved by mounting the collimated X-ray source 10 and/or the detectors 18 on a movable rotatable gantry. Each detector responds to one pencil-like beam from the collimated X-ray source but, during rotation of the gantry, obtains a multiplicity of different and discrete data sets relative to the amount of attenuation of the X-ray beam while passing edgewise through the plane of interest 14 within the patient 16. This information then is fed to either a general-purpose programmed computer 20 or a special-purpose computer device whereat, through well-known mathematical algorithms, the image through the plane or cross-sectional slice 14 of the patient is reconstructed. The reconstructed image is subsequently read-out or displayed by a display device 22 such as a cathode-ray tube screen, printer, or the like.
From a theoretical viewpoint, computer-assisted tomographic scanning techniques are capable of producing a very finely resolved reconstructed image of the cross-sectional plane or "slice" in question. Practical and technological problems, however, detract from this theoretical possibility and, to date, images obtained through known computer-assisted tomographic scanning devices still do not provide desired levels of resolution.
One particularly important and significant problem associated with the known state-of-the-art is poor or blurred resolution of the reconstructed image caused by patient movement during the X-ray scan. In this respect, and so as to enable an adequate number of statistical photon events to be detected and subsequently evaluated, it oftentimes takes several minutes for a single scan to be effected of a cross-sectional plane or "slice" through the patient's body. During this time period, it is absolutely essential that the patient be motionless in that normal breathing, heartbeat, and other physiological effects produce a blurred image. A slow scan time, therefore, cannot avoid a certain deterioration in image quality. On the other hand, a fast scan time, one in which a complete scan is effected in a matter of seconds as opposed to minutes, generally is not yet a practical reality due to equipment limitations such as slow response times, dosage restraints, and the like.
For example, to provide a sufficient number of statistical events so as to enable reconstruction of an image with suitable resolution, a minimum number of photons is required to be delivered by an X-ray beam. If these photons must be delivered in a shorter as opposed to a longer period of time, then current equipment requires that the intensity of the radiation beam be increased. Yet, an increase in intensity of the radiation beam can provide an unacceptably high patient dose. A weaker intensity of impinging radiation, however, may not provide enough photons into the actual plane of interest to obtain meaningful information and may be such that the information obtained is lost due to the signal to noise ratio of the detectors.
A further problem concerns image degradation due to the effect of scattering in the area immediately surrounding the patient cross-section of interest. In this respect, prior-art devices have not been capable of limiting the beam only to the thin area of interest, and thus have had to contend with increased scattering, and with the delivery of unnecessary radiation to the patient.
Many of these problems could be avoided and a computer-assisted tomographic system can achieve high resolution of the image with a fast scanning rate and at low dosage through more careful control and collimation of the beam of emitted X-ray photons. Such beam should be extremely narrow and tightly collimated, and have a very narrow angle of divergence or beam spread such that virtually all of the photons emitted by the beam are directed through only the area of the "slice" and into an appropriate detector. Thus, not only would resolution be improved in that the maximum number of photons passing through a particular object within the cross-section of the patient would be received by the detector, noise due to scattered radiation outside the particular angle of view would be reduced and, very importantly, the radiation dosage to which the patient is subjected outside the thin plane of interest would be maintained at an absolute minimum.
To date, existing technology has not been able to supply a beam possessing these desirable characteristics and thus, the development of a "fast" computer-assisted tomographic imaging device exhibiting high resolution and minimizing radiation dosage to the patient has been retarded.
Part of the technological problem resides in the fact that X or Gamma rays cannot readily be focused electro-magnetically or by other means. The sources of such radiation are basically isotropic and the rays can generally be directed by geometric collimation techniques utilizing appropriate attenuating materials. However, standard collimation geometries conventionally available in the art and now utilized in conjunction with existing computer-assisted tomographic imaging devices by their nature result in diverging beams and thus in peripheral areas outside of the given "slice" or cross-section of interest of the patient being exposed to unnecessary radiation, increasing the risk to the patient and further degrading the information received during a diagnostic or analytic procedure by scattering from outside the region of interest into the sensing or detecting mechanisms, thereby causing a loss of resolution and possibly even providing erroneous data.